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Mineralocorticoid receptor antagonists (MRAs) are commonly used to reduce blood pressure, left-ventricular hypertrophy, and urinary albumin excretion in patients with essential hypertension or primary aldosteronism. Effects of MRAs on hypertensive organ damage seem to occur beyond what is expected from the mere reduction of blood pressure. This suggests that activation of the mineralocorticoid receptor plays a central role in the development of cardiac and renal abnormalities in hypertensive patients. However, broad use of classic MRAs such as spironolactone has been limited by significant incidence of gynecomastia and other sex-related adverse effects. To overcome these problems, new aldosterone blockers have been developed with different strategies that include use of nonsteroidal MRAs and inhibition of aldosterone synthesis. Both strategies have been designed to avoid the steroid receptor cross-reactivity of classic MRAs that accounts for most adverse effects. Moreover, inhibition of aldosterone synthesis could have an additional benefit due to blockade of the mineralocorticoid receptor-independent pathways that might account for some of the untoward effects of aldosterone. The new aldosterone blockers are currently having extensive preclinical evaluation, and one of these compounds has passed phase 2 trials showing promising results in patients with primary hypertension and primary aldosteronism. This narrative review summarizes the knowledge on the use of classic MRAs in hypertension and covers the evidence currently available on new aldosterone blockers.

Clinica Medica, Department of Experimental and Clinical Medicine, University of Udine, Udine, Italy

INTRODUCTION

Spironolactone is a nonselective mineralocorticoid receptor antagonist (MRA) known to lower blood pressure since the early 1960s [1,2]. Spironolactone acts in the aldosterone-sensitive distal tubular site of the nephron by indirect inhibition of sodium reabsorption through the epithelial sodium channel and stimulation of potassium retention, being therefore classified among potassium-sparing diuretics (Fig. 1). Because of its nonselective binding to the mineralocorticoid receptor, spironolactone can antagonize the androgen receptor causing a variety of sexual adverse events in both men and women. Other molecules similar to canrenone, the active metabolite of spironolactone [3], were synthesized in the 1980s. Both prorenoate and mexrenoate, however, induced similar adverse effects and their use was abandoned.

In recent years, the more selective MRA eplerenone [4] has been developed with the intent to reduce the antiandrogenic effect. However, despite eplerenone being androgen receptor-inactive, it appears to be less powerful than spironolactone in reducing blood pressure [5,6]. Additional differences between spironolactone and eplerenone reside in their pharmacokinetic properties [7]. Spironolactone has a complex metabolism and a long half-life (greater than 12 h in healthy individuals, 24 h in heart failure patients, and up to 58 h in patients with cirrhotic ascites). In fact, spironolactone is converted in the liver to two active metabolites, 7α-thiomethylspironolactone and canrenone, which are responsible for persistence of the pharmacological effect. Conversely, eplerenone does not have active metabolites, its half-life at steady state is of 3–4 h, and it is directly metabolized in the liver by the cytochrome P450 member, CYP3A4, explaining why compounds that affect CYP3A4 function can change its blood concentration. Due to its short half-life, eplerenone should be administered at least twice daily.

The interest in MRAs has sensibly grown in recent years due to evidence clearly supporting the cardioprotective and nephroprotective properties of these agents in patients with heart failure and hypertension [8–10]. These organ-protective effects of MRAs are, at least in part, independent of the effects on blood pressure. This article summarizes the evidence on use of MRAs in endocrine and primary hypertension and focuses on the new pharmacological strategies that have been developed to block the activity of aldosterone.

MINERALOCORTICOID RECEPTOR ANTAGONISTS IN PRIMARY ALDOSTERONISM

In primary aldosteronism hypertension is caused by excess secretion of aldosterone from the adrenal cortex in the context of either unilateral [usually an aldosterone-producing adenoma (APA)] or bilateral [usually idiopathic adrenal hyperplasia (IHA)] disease of the adrenal glands. Spironolactone is the drug of choice for patients with bilateral IHA and although laparoscopic adrenalectomy is more cost-effective than lifelong medical therapy for unilateral disease [11]; this drug is recommended also for patients with unilateral disease who are unwilling or unable to undergo surgery [12]. Retrospective assessment of patients with APA who were treated with spironolactone for 5 years showed significant reduction of average blood pressure values (−46 mmHg for systolic, −27 mmHg for diastolic), although most patients required use of additional antihypertensive agents, and incidence of adverse effects (breast tenderness, cramps, decreased libido) was remarkable [13]. Efficacy of drugs in the treatment of primary aldosteronism has never been evaluated in randomized, placebo-controlled trials, but observational studies and extensive clinical experience clearly demonstrate that in these patients spironolactone effectively reduces blood pressure and corrects hypokalemia [14]. Use of spironolactone in IHA at daily doses from 50 to 400 mg induces an average blood pressure reduction of 25% for systolic and 22% for diastolic [12]. Effective reduction in blood pressure could be obtained also with lower doses (25–50 mg/day) of spironolactone, thus limiting the burden of the dose and sex steroid-related adverse effects of this compound [15]. Alternative strategies that, despite lack of trial evidence, can be considered to limit spironolactone-related adverse effects in primary aldosteronism, are either the use of its companion compound potassium canrenoate or other potassium-sparing diuretics such as amiloride and triamterene [12].

Eplerenone has been synthesized in the attempt to obtain a more selective inhibition of the mineralocorticoid receptor and to overcome the adverse effects due to cross-reaction of spironolactone and canrenoate with androgen receptors. Two recent studies have compared the efficacy of eplerenone and spironolactone on blood pressure reduction in patients with primary aldosteronism. In a first prospective, randomized, open-label study, eplerenone (from 50 to 200 mg/day) and spironolactone (from 50 to 400 mg/day) were administered for 24 weeks to 34 patients with IHA, resulting in an equally effective reduction in blood pressure [16]. Another multicentric, parallel-group, double-blind study randomized 141 primary aldosteronism patients to be treated with eplerenone or spironolactone for 16 weeks, using a titration-to-effect approach (respectively, eplerenone from 100 to 300 mg/day and spironolactone from 75 to 225 mg/day) [6]. The decrease in SBP and DBP was greater in patients treated with spironolactone, with a difference that was already significant at the fourth week of active treatment. In this study, despite a higher incidence of gynecomastia/mastodynia and hyperkalemia in the spironolactone group, the overall incidence of adverse events was comparable in the two treatment groups.

Normalization of blood pressure is not the only goal of treatment of primary aldosteronism and, as for all types of hypertensive conditions, effective prevention of clinical and subclinical organ complications is mandatory. Because of the demonstration that excess aldosterone is associated with a variety of cardiovascular [17,18], renal [19,20], and metabolic [21,22] sequelae that reflect the capability of aldosterone to induce organ damage over that induced by hypertension itself [23], this goal achieves specific relevance in patients with primary aldosteronism. Long-term follow-up studies have demonstrated that, in addition to being effective in reduction of blood pressure, treatment of primary aldosteronism with spironolactone reduces the rate of cardiovascular and renal complications, decreases left-ventricular mass and urinary protein excretion, and corrects abnormalities of glucose metabolism [14].

Cardiovascular outcomes were compared in patients with primary hypertension and patients with primary aldosteronism who had comparable cardiovascular risk profile, but greater retrospective incidence of coronary artery disease, cerebrovascular events, and sustained arrhythmias [24]. Patients were followed for an average of 7.4 years after treatment during which occurrence of a combined cardiovascular endpoint was comparable in the two groups. Analysis of primary aldosteronism patients treated with adrenalectomy or spironolactone did not reveal significant difference, indicating that surgical and medical treatment of primary aldosteronism is equally valuable in the prevention of cardiovascular events. In the same cohort we investigated the long-term outcomes of renal function by measuring the rates of change of glomerular filtration and albuminuria [25,26]. After an initial decrease in creatinine clearance and urinary albumin excretion that occurred in primary aldosteronism patients due to reversal of the aldosterone-induced intrarenal hemodynamic adaptation [27], changes in glomerular filtration and albuminuria in the long term were comparable to those of patients with primary hypertension. Notably, patients with primary aldosteronism who were treated with spironolactone had the same renal outcomes as those treated with surgery, indicating that spironolactone is as effective as surgery in the correction and prevention of renal complications of primary aldosteronism.

In addition to evidence supporting the beneficial role of spironolactone in the long-term cardiovascular and renal protection of patients with primary aldosteronism, it is known that chronic use of this agent can affect positively also some subclinical conditions that anticipate major events and appear to have specific relevance in primary aldosteronism. Echocardiographic studies have demonstrated the presence of an excess increase of left-ventricular mass in patients with primary aldosteronism as compared to patients with primary hypertension [28–32], indicating that elevated aldosterone increases left-ventricular mass beyond the amount needed to compensate the blood pressure-related cardiac afterload. Two long-term follow-up studies have reported that treatment of patients with primary aldosteronism with spironolactone decreases left-ventricular mass similar to adrenalectomy [31,33]. Also, treatment with spironolactone reduces albuminuria in primary aldosteronism [25,26] and halts the progression of renal cystic disease that has been demonstrated to be highly prevalent in primary aldosteronism [34]. Finally, treatment with spironolactone has been demonstrated to correct glucose metabolism abnormalities, including hyperinsulinemia and insulin resistance, that are found in patients with primary aldosteronism [35].

In summary, currently available evidence clearly supports the concept that, when used in patients with primary aldosteronism due to bilateral adrenal disease, spironolactone effectively reduces blood pressure, corrects hypokalemia, and prevents cardiovascular and renal complications. To date, experience on treatment of primary aldosteronism with eplerenone is limited to a couple of studies with the best evidence, suggesting that this drug is less powerful than spironolactone in lowering blood pressure. Data on use of MRAs in patients with primary aldosteronism and unilateral adrenal disease are as yet insufficient to draw conclusions; therefore, adrenalectomy remains the treatment of choice for these patients.

MINERALOCORTICOID RECEPTOR ANTAGONISTS IN PRIMARY HYPERTENSION

Although, for obvious reasons, use of spironolactone has been historically predominant in conditions characterized by aldosterone excess, this agent has also been used for decades for treatment of primary hypertension, and use of aldosterone receptor antagonists in these patients has been discussed in comprehensive review articles [36,37]. Despite spironolactone being available for more than 50 years, there is still an amazing shortage of well controlled trials on the use of this compound in primary hypertension. This could be explained, at least in part, by its limited selectivity for mineralocorticoid receptor that leads to progesterone and androgen-dependent adverse effects: from breast engorgement to gynecomastia, from loss of libido to impotence, from menstrual irregularities to amenorrhea. The interest in aldosterone as a target for antihypertensive therapy was revived with the development of eplerenone. This agent has been tested in patients with primary hypertension and in patients with congestive heart failure, reporting less pronounced sexual adverse effects.

The antihypertensive effects of spironolactone have been overviewed in a recent Cochrane meta-analysis that has included five cross-over studies and one randomized controlled trial with a total of 179 primary hypertensive patients who were followed from 4 to 8 weeks [38]. This meta-analysis has shown that spironolactone decreases SBP and DBP by 20 and 7 mmHg, respectively, but this effect is reached with doses between 100 and 500 mg/day. With these doses, the risk of hyperkalemia is an important limitation and this is why use of spironolactone in the treatment of primary hypertension has been limited to combination with other types of diuretics that induce urinary potassium losses.

Despite the fact that the effects of spironolactone on blood pressure in primary hypertensive patients are modest, possible benefits on subclinical hypertensive organ damage that might be obtained with lower doses of the drug should be considered. Initial studies suggested that 50 mg/day of canrenoate improve left-ventricular diastolic function in primary hypertensive patients with diastolic dysfunction, an effect that occurred independent of blood pressure and left-ventricular mass changes [39]. Some small studies conducted in patients with hypertension-induced left-ventricular hypertrophy have reported that addition of spironolactone to blockers of the renin–angiotensin system increases the effects on left-ventricular mass reduction [40–42] and are summarized with other studies [43–48] on the effects of MRAs on left-ventricular hypertrophy in Table 1. Overall, current evidence indicates that spironolactone could have a place in the treatment of primary hypertensive patients with left-ventricular hypertrophy and/or diastolic dysfunction.

Also eplerenone has been repeatedly tested in patients with primary hypertension. One uncontrolled, open-label study [49] and many randomized controlled trials have reported the antihypertensive effect of eplerenone when compared to either placebo [5,50–52] or other classes of antihypertensive agents [5,45,53–56]. These studies are summarized in Table 2. Controlled studies have compared the blood pressure-lowering effects of eplerenone at doses comprised from 50 to 400 mg/day with those of either placebo, enalapril, losartan, amlodipine, or spironolactone. In these studies, duration of follow-up was from 2 to 14 months. In all studies, eplerenone was more effective than placebo in reducing blood pressure; in two studies it was more potent than losartan, and in the remaining studies the hypotensive effects were comparable to those of enalapril, amlodipine, and spironolactone.

Eplerenone has been demonstrated to be beneficial also on hypertension-related subclinical organ damage (Table 1). In the 4-E Left Ventricular Hypertrophy Study, regression of left-ventricular hypertrophy was compared in primary hypertensive patients who were treated with eplerenone, enalapril, or their combination for 9 months [45]. Left-ventricular mass index decreased significantly and comparably in patients treated with eplerenone or enalapril, whereas the combination of the two agents showed additive effects on left-ventricular mass reduction. In other studies, eplerenone was reported to be more effective than amlodipine [54], enalapril [55], or losartan [56] in decreasing urinary albumin excretion.

Despite the fact that eplerenone has a satisfactory tolerability and safety profile even at the highest doses that have been clinically tested (200 mg/day) [49], some studies indicate that its effects on blood pressure in primary hypertension [5] are inferior to those of spironolactone. This, in addition to the high costs of production, means that eplerenone has limited use in several countries where spironolactone and potassium canrenoate are still the only MRAs approved for clinical use. In Europe, eplerenone is currently marketed only in some countries and only with the indication of heart failure, thus limiting its use in clinical research.

MINERALOCORTICOID RECEPTOR ANTAGONISTS IN RESISTANT HYPERTENSION

Resistant hypertension is defined by failure to reduce blood pressure below 140/90 mmHg with the use of at least three effective antihypertensive agents including a diuretic [57]. Several clinical case reports have suggested that spironolactone can be useful in the treatment of resistant hypertension and, in particular, in hypertension associated to obesity and/or obstructive sleep apnea syndrome [58–61]. One controlled [62] and many noncontrolled prospective [59] and retrospective [63–68] studies have confirmed that addition of 25–50 mg/day of spironolactone to current treatment effectively reduces blood pressure in patients with resistant hypertension. In the ASPIRANT (Addition of SPironolactone In patients with Resistant Arterial hypertension) trial [62], 117 patients with resistant hypertension were randomized to treatment with spironolactone or placebo in a double-blind protocol. The trial was prematurely stopped after the first interim analysis because of a significant reduction of SBP in patients taking spironolactone as compared to those taking placebo [−5.4 mmHg; 95% confidence interval (CI) 0.8–10.0; P = 0.024]. Notably, the average BMI of the study population in this trial was 32.3 kg/m2, clearly indicating that patients were either obese or overweight. Although inclusion of incident cases of primary aldosteronism in this study might have affected the results, in this study the only predictor of blood pressure response to spironolactone was baseline plasma aldosterone-to-renin ratio. In the prospective, uncontrolled study by de Souza et al. [60], 175 Brazilian patients with resistant hypertension were treated with 25–100 mg/day of spironolactone and, after a median interval of 7 months, 24-h SBP and DBP decreased by 16 mmHg (95% CI 13–18; P < 0.001) and 9 mmHg (95% CI 7–10; P < 0.001), respectively. The baseline characteristics of these patients (BMI 30.2 ± 5.1 kg/m2) showed again that they were either overweight or obese and had high prevalence of diabetes (33%), dyslipidemia (86%), left-ventricular hypertrophy (76%), and previous cardiovascular diseases (52%). In the ASCOT-BPLA (Anglo-Scandinavian Cardiac Outcomes Trial-Blood Pressure Lowering Arm) trial, Chapman et al.[64] analyzed retrospectively those patients who took spironolactone as fourth-line therapy because of resistant hypertension. In these patients, spironolactone at a median dose of 41 mg/day significantly reduced blood pressure by 22/10 mmHg (95% CI 21–23 and 9–10; P < 0.001). Even in this study, patients with resistant hypertension had significantly higher BMI (29.4 ± 4.6 vs. 28.7 ± 4.6 kg/m2; P < 0.001), and prevalence of diabetes (40 vs. 27%; P < 0.001) and left-ventricular hypertrophy (27 vs. 22%; P < 0.001) than patients who were not resistant to treatment. Very similar results were reported in patients of different geographical areas in the retrospective studies by Engbaek et al.[63], Nishizaka et al.[65], Ouzan et al.[66], Sharabi et al.[67], and Lane et al.[68].

The reasons for the beneficial effect of spironolactone in patients with resistant hypertension are not clear, although this effect suggests a substantial contribution of aldosterone to maintenance of increased blood pressure despite treatment. In fact, inappropriate secretion of aldosterone has been reported in hypertensive patients with associated obesity and/or obstructive sleep apnea syndrome. Also, it is a well established notion that aldosterone can escape the inhibitory effects of renin–angiotensin system blockers in patients treated with these drugs, thereby leading to a form of hypertension that is largely aldosterone-dependent [69]. Finally, it cannot be excluded that, at least in some cases, resistance to antihypertensive treatment hides a form of primary aldosteronism.

PROTEINURIC NEPHROPATHIES AND OTHER POTENTIAL USES OF MINERALOCORTICOID RECEPTOR ANTAGONISTS

Proteinuria represents an early marker of renal damage, causes progression of renal disease by itself [70], and is associated with an increased cardiovascular risk [71]. A relationship between elevated plasma levels of aldosterone and deterioration of renal function was noted in initial studies conducted in patients with advanced renal failure [72,73], suggesting that aldosterone might contribute to progression of renal damage. More recently, renal biopsy studies of proteinuric patients have demonstrated increased expression of the mineralocorticoid receptor mRNA in the kidney, suggesting that increased activation of the receptor may contribute to progression of glomerular damage [74]. Possible benefits of MRAs in proteinuric nephropathies have been investigated in small and relatively short-lasting controlled clinical trials that have been conducted in patients with diabetic nephropathy [75–78] or other proteinuric diseases [79–82]. In these studies, either spironolactone (25 mg/day) [75,76,78–82] or eplerenone (50–100 mg/day) [77] have been used on top of treatments that included one or two additional blockers of the renin–angiotensin–aldosterone system (RAAS). Overall, these trials support the possibility that MRAs provide additional benefit to other RAAS blockers in reducing urinary protein excretion and their results have been extensively reviewed by Briet and Schiffrin [83]. Thus, although interventional studies on use of MRAs in patients with diabetic nephropathy or other types of chronic kidney disease are of limited size, these studies have reported promising results with a significant reduction of proteinuria. Larger trials that will test the efficacy and safety of MRA in chronic kidney diseases are needed.

Because of the growing evidence of an involvement of mineralocorticoid receptor in a number of pathophysiological mechanisms related to common diseases, clinical research is now testing new potential uses of MRAs (http://clinicaltrials.gov/ct2/results?term=spironolactone&recr=Open&no and http://clinicaltrials.gov/ct2/results?term=eplerenone%26amp;recr=Open&no_unk=Y). Phase 2, 3, and 4 clinical trials are currently recruiting patients to investigate the effects of spironolactone and/or eplerenone in a number of pathological conditions that include acute myocardial infarction, heart failure with preserved ejection fraction, hypertrophic cardiomyopathy, adult congenital heart disease, pulmonary hypertension, hypertension with autonomic failure, secondary prevention of stroke, calcineurin-related nephrotoxicity, dialysis, metabolic syndrome, glucose intolerance, and puberal overweight. The results of all these trials should be available within a few years.

ADVERSE EFFECTS OF MINERALOCORTICOID RECEPTOR ANTAGONISTS

Adverse effects of spironolactone are well known; however, no systematic analysis has been done to assess the clinical impact of such effects. The analysis of Batterink et al.[38] included trials on use spironolactone in hypertension that were performed over almost 50 years and concluded that the evidence supporting differences between spironolactone and placebo on mortality, morbidity, serious adverse events, withdrawals due to adverse effects, or total adverse effects was insufficient. Although randomized controlled trials or systematic analyses could not provide conclusive information on the clinical impact of adverse effect of spironolactone in hypertension, some data have been extrapolated from nonrandomized observational trials. In two case–control [84,85] and one cohort studies [86], spironolactone was associated with an increased rate of upper gastrointestinal bleeding and an observational study on 182 hypertensive patients showed that spironolactone (average dose 96.5 mg/day for 23 months) increased plasma potassium and creatinine levels by an average of 0.6 mmol/l and 0.09 mg/dl, respectively [87]. In the latter study, gynecomastia developed in 13% of men using spironolactone alone or in combination with other antihypertensive drugs.

Despite clear evidence of dose-related adverse effects of spironolactone, significant incidence has been reported also at the lowest doses of 25–50 mg/day. In the heart failure patients who were included in the Randomized Aldactone Evaluation Study (RALES), spironolactone induced a significant increase in serum potassium and creatinine levels (0.3 mmol/l and 0.055 mg/dl, respectively), but the incidence of severe hyperkalemia (≥6.0 mmol/l) did not differ from controls [8]. Most important, however, the incidence of gynecomastia/breast engorgement causing drug discontinuation in patients treated with spironolactone was 10-fold than that of controls. Because of these effects of spironolactone on the breast, the potential risk of a breast cancer promotion by MRAs has also been supposed. On this point, reassuring results come from a recent large retrospective cohort analysis of more than 1 million of women older than 55 years of age included in the General Practice Research Database in the UK. In this analysis, the incidence of breast cancer during a mean follow-up of 4.1 years in about 28 000 women exposed to spironolactone for cardiovascular reasons from 1987 and 2010 was not different from that of a control group of unexposed women registered with the same practice and matched by year of birth and socioeconomic status [88].

Moreover, concerns about the potential renal toxicity of spironolactone have been raised because the rapid increase in use of this drug that occurred after publication of the RALES was associated with a marked concurrent increase in hospital admissions and deaths due to hyperkalemia [89]. A recent population-based longitudinal analysis on use of spironolactone and hyperkalemia was done in Scotland [90], showing that despite a marked increase in the frequency of use in patients with and without heart failure, hospital admission for hyperkalemia and cases of outpatient hyperkalemia did not increase. Appropriate monitoring of serum creatinine and potassium concentrations is of primary importance in the prevention of renal adverse effects of spironolactone [90], although additional predictors of hyperkalemia and renal failure should be considered. These predictors of hyperkalemia include older age, higher baseline serum potassium levels, concomitant treatment with β-blockers [91], or trimethoprim-sulfamethoxazole [92], whereas renal failure is facilitated by concomitant use of thiazide diuretics [91]. With specific reference to renal complications, eplerenone does not show a better safety profile than spironolactone [6] (Table 2), although it is significantly less likely to cause sex-related adverse effects in both men and women. Frequency of adverse effects of spironolactone in a selection of clinical studies [8,13,64,87,91,93] is reported in Table 3.

NEW ALDOSTERONE BLOCKERS

Promising results with MRAs in the treatment of hypertension and prevention of hypertension-related organ damage have prompted the search and possible development of new aldosterone antagonists. Search has followed two main strategies: the first has consisted in the development of nonsteroidal MRAs that could overcome the adverse effects of spironolactone and canrenoate without losing the pharmacological properties of these compounds; the second has aimed at developing drugs that inhibit aldosterone biosynthesis and has resulted in the generation of aldosterone-synthase direct inhibitors.

The first strategy moved its initial steps from the demonstration that some dihydropyridine calcium channel blockers (CCBs) exert also a mineralocorticoid receptor antagonist activity [94–96]. Researchers of the Pfizer laboratories showed that these dihydropyridine CCBs, namely nimodipine, felodipine, and nitrendipine, block aldosterone-induced mineralocorticoid receptor activation by competing with aldosterone binding to the mineralocorticoid receptor ‘ligand-binding domain’ (LBD) [94]. This competition for mineralocorticoid receptor binding decreases the aldosterone-mediated recruitment of transcriptional co-regulators that are primary for mineralocorticoid receptor-related activation of DNA transcription. The affinity of dihydropyridines for the LBD is lower than for the L-type calcium channels, indicating that inhibition of mineralocorticoid receptor activity is independent of the effects on calcium channels. The efficacy of dihydropyridines as mineralocorticoid receptor antagonist is comparable to that of eplerenone [94], whereas nondihydropyridine CCBs such as verapamil and diltiazem do not have any mineralocorticoid receptor inhibitory activity. The molecular reasons for recognition of both L-type calcium channels and mineralocorticoid receptor-LBD by dihydropyridines are not clear; however, the chiral analysis of the 1,4-dihydropyridine mebudipine indicates that the CCB activity and mineralocorticoid receptor antagonism reside in opposite enantiomers of the molecule [95]. On this basis, many drug companies have patented new molecules in the dihydropyridines family with mineralocorticoid receptor antagonist properties (Takeda Pharm.: WO/2005/097118; Bayer: DE102005034267). Chemical optimization of mineralocorticoid receptor antagonist activity of dihydropyridine compounds led Bayer to develop one of the first nonsteroidal mineralocorticoid receptor antagonists (BR-4628) that shares a similar efficacy with spironolactone as mineralocorticoid receptor inhibitor, but has no effect on the other steroidal receptors and on the L-type calcium channel [97].

The other new class of antialdosterone agents includes the selective aldosterone synthase inhibitors (ASIs) that have been overviewed recently [98]. Aldosterone synthase or CYP11B2 is an enzyme of the cytochrome P450 family with steroid 18-hydroxylase, 18-oxidase, and 11-beta-hydroxylase properties. It catalyzes the formation of aldosterone from 11-deoxycorticosterone within the zona glomerulosa in the adrenal cortex [99] (Fig. 1). CYP11B2 deficiency in humans is characterized by a low/absent aldosterone synthesis in the adrenal cortex and high plasma renin activity, and is responsible for a sodium-wasting phenotype associated with retarded growth [100]. Conversely, enhanced activation of CYP11B2, as it is supposed to occur in the polymorphism 344C/T, is associated with increased left-ventricular mass [101] and greater risk to develop hypertension [102]. Because CYP11B2 activity is the limiting biochemical step in aldosterone synthesis, its selective inhibition is a good target for prevention of aldosterone untoward effects mediated by both mineralocorticoid receptor-dependent and mineralocorticoid receptor-independent pathways.

At present, only two compounds (both synthesized by Novartis) with selective CYP11B2 inhibitory properties have been tested in animal models [103–107] and, more recently, in the clinical setting [108,109]. FAD286 is the D-enantiomer (stereoisomer that rotates to the right the plane-polarized light) of fadrozole, an aromatase inhibitor developed to treat advanced breast cancer that reduces aldosterone levels and increases plasma renin activity in rats fed either low or high-sodium diet [103]. In a transgenic rat model of secondary hypertension in which the angiotensinogen gene is overexpressed and circulating angiotensin II levels are increased, oral administration of FAD286 has reduced mortality by four times. In transgenic rats treated with FAD286, cardiac hypertrophy, albuminuria, and histological evidence of glomerular damage were less frequent than in control animals [104]. Despite very minor effects of FAD286 on blood pressure, its effects on organ damage were comparable to those of the angiotensin receptor blocker, losartan. Similar results were obtained in uninephrectomized rats fed a high-sodium diet and that were treated with angiotensin II to induce renal fibrosis. In this model, FAD286 decreased both cardiac and renal hypertrophy and fibrosis without affecting blood pressure levels [105]. In another study conducted in rats with experimentally induced myocardial infarction, FAD286 had effects on prevention of left-ventricular remodeling and systolic dysfunction that were comparable to those of spironolactone [106]. In a rodent model of accelerated atherosclerosis (apolipoprotein E-deficient mice), FAD286 reduced the severity of atherosclerotic plaques and the expression of several inflammatory markers without affecting plasma aldosterone levels [107].

Following encouraging preclinical results, Novartis tested the first ASI LCI699, in two phase 2 clinical trials. LCI699 is similar in structure to FAD286 and it has been developed specifically for human use, although its inhibition of CYP11B2 is nonselective since this compound partially inhibits also 11-beta-hydroxylase (CYP11B1), the enzyme responsible for the final step in cortisol biosynthesis. In a first trial, LCI699 was administered to 14 patients with primary aldosteronism and effects were compared to those of placebo. After 2 weeks of treatment, there was a dose-dependent decrease in plasma aldosterone and an increase in 11-deoxycorticosterone, potassium, and adrenocorticotropin (ACTH) levels. Treatment induced mild reduction of 24-h ambulatory SBP (−4.1 mmHg) after 4 weeks [108]. More recently, Calhoun et al.[109] have published the first randomized, double-blind, placebo-controlled, phase 2 trial with LCI699 that was conducted in 524 patients with primary hypertension. In this trial different doses of LCI699 have been tested and all doses have induced significant decrease in 24-h SBP, whereas only the highest dose has reduced 24-h DBP. The tolerability profile of LCI699 was similar to that of eplerenone, but in approximately 20% of patients treated with this compound ACTH-induced cortisol release was blunted suggesting inhibition of glucocorticoids synthetic pathway. This effect could be ascribed to the inhibitory effect on CYP11B1, and its clinical consequences could be either detrimental, because of the importance of ACTH-activated cortisol production in adaptation to stress, or beneficial in conditions such as hypercortisolism. The clinical relevance of these effects of LCI699 will deserve further evaluation, and development of more selective ASI may overcome the concern related to the inhibition of cortisol synthesis. Along this line, chemical manipulations of metyrapone has made available a series of N-(pyridin-3-yl)benzamide derivatives that have been shown to inhibit selectively CYP11B2 in vitro and will have to be tested for metabolic stability and in-vivo effects [110]. In the meantime, phase 2 clinical studies are testing LCI699 in patients with primary hypertension, resistant hypertension, primary aldosteronism, and Cushing's disease (http://clinicaltrials.gov/ct2/results?term=LCI699).

In conclusion, recent evidence indicates that in primary aldosteronism excess aldosterone is associated with organ damage over that induced by hypertension itself. Aldosterone levels contribute also to the development and progression of cardiovascular and renal damage in patients with primary hypertension. Although use of steroidal antagonist of mineralocorticoid receptor has proven to be beneficial on blood pressure and hypertensive organ damage, high rates of steroid receptor-related adverse effects limit their clinical use. To overcome these effects, nonsteroidal MRAs and ASIs have been developed and patented recently. Eplerenone has good tolerability and safety profile even at the highest doses that have been tested clinically, but some studies indicate that its effects on blood pressure are inferior to those of spironolactone. Some dihydropyridine CCBs have been shown to act as mineralocorticoid receptor antagonist and new agents derived from these compounds are going to be tested in clinical trials after preclinical validation. At present, only the ASI LCI699 has been tested in initial clinical trials and encouraging results on blood pressure reduction have been reported in patients with primary hypertension and primary aldosteronism. Further laboratory, animal, and human research will be needed before new drugs that block the effects of aldosterone will be made available for everyday clinical use.

ACKNOWLEDGEMENTS

Conflicts of interest

Reviewer's Summary Evaluation Reviewer 2

The authors provide a comprehensive discussion of the use of current and experimental classes of agents for blocking and/or interrupting mineralocorticoid receptor activation. Clinical use of current mineralocorticoid receptor antagonists, including spironolactone and eplerenone, is discussed in detail as are emerging classes, such as non-steroidal mineralocorticoid antagonists and aldosterone synthase inhibitors. The summary is well written and provides a valuable overview of the topic that will be of use both to clinicians using these agents for routine care and to investigators interested in the state-of-the art in this important area.